Magnetic field detector



nieuwe Apr. s, 194s MAGNETIC FIELD DETECTOR David H. Cunningham, Haddondeid, andllcrbert Belar, Palmyra, N. J., aasignors to Radio Corporation oi.' America, a corporation of Delaware Application November 2, 1942, Serial No. 464,296 1 Claim. (Cl. 175-183) i This invention relates generally to magnetic field detectors and particularly to a new and improved method of and means for detecting the presence ofr or variations in, a magnetic field by measuring the distortion caused by this field in yan electromagnetic circuit.

It has been found that in currents which flow through a magnetic circuit which includes a magnetic core, such vas in a transformer, relatively high even numbered harmonics can hev generated if the core is subjected to a magnetic field. If the core is relatively long and thin and is of high permeability material, Weak magnetic fields, or changes therein, can be detected and measured. Thiseifect may be utilized for measuring or indicating variations in the earths magnetic field. It may also be employed for indicating` the direction and magnitude of the earths magnetic field, or any other magnetic field.

Briefly, the apparatus required comprises a re1- atively long, thin core of high permeability magnetic material such as Mu-metal. The core is surrounded by one or more windings. -A source of alternating current having a frequency, for example, of five hundred cycles per second, which is preferably free of second harmonics, is connected to one of the windings surrounding the core. A second winding is provided on the core and connected through, for example, a thousand cycle iilter network to a suitable indicator for 5 measuring the second harmonic component.

The 1000 cycle filter network is connected with its input to another winding and its output to aA suitable indicator, for measuring the -1000 cycle component. A single winding may he used in place of a transformer and the second harmonic current may be derived by any other known manner. If the lcore with its coil is so mounted that it can be rotated with respect to the magnetic eld, then it will be found that if the core is athwart or at right angles to the vector of the field, thatlno second harmonic will be detected othenthanwhich may be due to the generator itsel'flfffASthe device is turned from its null po sition, the amount of second harmonic output will increase, and a maximum indication will be obtained when the core is parallel to the vector of the field. Variations in the magnetic field will provide relatively large variations in second harmonic output up to a point when the core begins to saturate. From then on, further increases in the field will produce only small changes in the indicated second harmonic output.

By proper proportioning of the core and proper .choice of the material thereof, a maximum sensitivity for any given eld strength can be obtained. It is also possible to increase the sensitivity of the device to small changes in field strength by designing the core for measuring fields of the magnitude of the change and bringing -the total field to be measured within that range by passing a current through a coil or otherwise producing a bucking field of the proper amount. It should also be understood that vacuum tube amplifiers may be used to amplify the effect produced to any desired magnitude before filtering, or before or after detection. Also that amplifiers and filter functions may be combined in a selective amplfler.

Among the objects of the invention are to provide 'an improved method of and means for measuring the intensity of a magnetic field. Another object is to provide a new and improved method of and means for measuring variations in the strength of a magnetic field. Another object is to provide a new and improved method of and means for indicating the direction of a magnetic field. A further object is to provide a new and improved method of and means for measuring the characteristics of a magnetic field which includes energizing a core of high permeability material by currents from a source oflow frequency alternating potential, and measuring the even harmonic componentgenerated in the magnetic circuit by the magnetic field. A still further object of the invention is to provide'a plurality of such magnetic devices disposed in angular relation to each other, and means for measuring the even harmonic currents generated by the individual units to derive the direction and intensity of a magnetic eld. A further object of the invention is to provide means for orient-ing a single magnetic unit in a unidirectional eld for determining the direction of said field. A further object of the invention is to provide means for measuring the intensity of the magnetic field, irrespective of the direction, by measuring the sum of the square of three mutually normal vectors of the field.

The invention will be described by reference to the accompanying drawing, of which. Figure l is an elevational View of one embodiment thereof, and Figure 2 is a schematic diagram of a second embodiment. Similar reference numerals are applied to similar elements throughout the drawing. i y

Referring to Figure 1, a core of high permeability magnetic materal I is pivoted at its center 2, and supported in a substantially horizontal position by a vertical supporting member 3. y, A

3 primary winding 4 surrounds one armof the magnetic core I. The primary winding 4 is connected to a source of alternating potential such as, for example, a. nve hundred cycle generator 5 of conventional design. A .source .of bias potential, such as a battery B, may be connected between the winding 4 and the A.C. source 5. for the purpose of producing a bucking fleld to oppose any desired portion ofthe external field. Such an arrangement provides increased sensitivity for the measurement of minute iield strength variations. A secondary winding 6 surrounds the remaining arm of the pivoted core I. The secondary winding -6 is connected to the input of a one thousand cycle lter network 'I of conventional design, which rejects substantially all frequencies except the second harmonic of the five hundred cycle generator 5. The output of the lter network -1 is connected to any suitable indicator 8 for measuring the amplitude of the second harmonic component thus derived. As the position of the magnetic core I is varied both vertically and horizontally, the second harmonic indication on the meter 8 will approach a maxi- .mum when the axis of the magnetic core coincides with the direction of the unidirectional magnetic field.

Referring to Figure 2, three magnetic cores I, II, and 2|, which may be of the type described heretofore, each include primary and secondary windings of similar design. The magnetic core I includes the primary winding 4 and the secondary winding 6. The second magnetic core II is disposed normal to the first magnetic core I, and includesthe primary Winding I4, and the secondary winding I6. The third magnetic core 2l is disposed normal to both the first and second magnetic cores I and I I, and includes the primary winding 24 and the secondary winding 26. A five ,hundred cycle generator 5, of a conventional type, through proper isolating or decoupling means I3, 23, and 33, is connected to energize all three of the primary windings, or separate generators Ill, and 30 may be used.

, For the purpose of illustration only, three double pole-double throw switches I2, 22 and 32 are used to change from the single to the separate A.C. generators. With all of the switches in the left hand position, separate generators are connected toseparate primary windings. With all of the switches in the right hand position, a single generator is connected through the separate isolating circuits I3, 23 and 33 to the windings 4, I4 and 24 respectively. The isolating circuits, which may comprise conventional buffer thermionic amplifiers, yprevent harmonic reaction between the various windings. The secondary winding 6 on the first magnetic core I is connected tothe primary of a rst coupling transformer I5. The secondary winding I6 on the second core II is connected to the primary of a second coupling transformer 25. The secondary winding 26, on the third core 2l, is connected to the primary winding of a third coupling transformer l35.

Thev secondary of the first coupling transformer I5 is connected through a. one thousand cycle filter 'I to the input of a detector 9. The secondary of the second coupling transformer 25 is connected through a second one-thousand These detectors are so designed that the output therefrom is proportional to the square of the field intensity to be measured. For instance, if a linear relationship existed between second harmonic output and field intensity, then this detector should be a square law detector. Actually over the most sensitive working range, the harmonic output varies as a higher than second power ofthe field intensity. The conversionof that by means of a detector to a square law function is well known in the art. Most common crystal or thermionic tube detectors have squarelaw characteristics for low appliedsignal magnitudes. Such square law detectors are described and explained, for example, in the Radio Engineers Handbook by F. E. Terman (McGraw-Hill, 1943) at pages 565-566, and in numerous other publications including those enumerated therein. The rectified outputs of the three detectors 9, I9, and 29 are combined additively and applied to the meter 8, which may be of any conventional type having the desired sensitivity. This meter indicates the algebraic sum of the squares of the field intensity along the vectors X, Y and Z. If desired, similar separate meters I8, 28, and 38 may be connected to the output circuits of each` of the detectors to indicate separately the output currents or voltages of the individual detectors. The first meter 8 will therefore indicate the square of the intensity of the total unidirectional magnetic field, and can`be calibrated to indicate intensity directly as a scalar quantity, 'while the indications on the other meters I 8, 28, and 38 may lbe utilized to indicate the respective squared values on the three rectilinear coordinates X-X', Y'-Y', Z-Z', of the unidirectional magnetic field.

It should be understood that the physical characteristics of the magnetic cores and windings will be determined by the characteristics of the magnetic iield under observation and by the sensitivity desired. The sensitivity of either of the modications described heretofore may be increased to any desired extent by the insertion of amplifiers, not shown, between the one-thousand cycle filters and the detectors. An idea of the sensitivity of the device without amplification may be had from the fact that the earths unidirectional magnetic field will provide a second harmonic current output of approximately ten percent of the amplitude of the applied five hundred cycle currents. l For example, a core of Mu-metal 2% inches long, 1/24 inch wide, and .015" thick, upon which are mounted two windings of 1`000`turns of No. 39 wire, will provide approximately .2 volt of second harmonic frequency when a 500 cycle signal of 2 volts is applied, and the core ls subjected to the earths magnetic field.

We claim as our invention:

A magnetic field detector including three similar cores ofhigh permeability magnetic material fixedly disposed normal to each other/in said field, a first winding on each of said cores, three separate similar so'urces of alternating current of substantially sine wave form, means for applying said current to each of said first windings from a different one of said sources, a second winding on each of Asaid cores, a plurality of filter networks for deriving currents of a selected even harmonic of said applied currents, means connecting a separate one 0f said filter networks to each of said second windings, the amplitude of said selected even harmonic currents being a function of the strength of said eld on the axes of each of said cores, three separate square-law detectors. means for applying the output currents from each of said il-lters to a diierent one of said detectors, means for indicating separately the output currents of each of said detectors, each of said output currents being proportional respectively to the square oi' the eld intensity on the axis of different ones of said cores, means serially connecting said detectors for combining algebracally said detector output currents, and means for indicating said combined currents in terms of the algebraic sum of the squares of the field intensities on the axes o1' said cores.

DAVID H. CUNNINGHAM. HERBERT BELAR.

6 REFERENCES CITED UNITED STATES PATENTS;

Number Name Y Date 1,546,427 Affel 1 July 21, 1925 2,047,609 Antranikian July 14, 1936 2,053,154 La Pierre Sept. 1, 1936 2,192,242 Robinson Mar. 5, 1940 2,213,357 Barth Sept. 3, 1940 FOREIGN PATENTS Number Country Date 451,850 Great Britain Aug. 10, 1936 

